Each chromatic hue is generally specified by the proportions of four unique hues (R, G, Y, and B) it contains. These proportions have been mainly established for spectral colours. We determined them for non-spectral colours of different excitation purity. Specifically, we have established isochromatic curves (isochromes) in the equiluminant plane for three trichromats. By isochrome we mean a set of points in the equiluminant plane which have the same amount of one of four unique hues. For example, a R-isochrome is a curve in the equiluminant plane all points of which represent colours having the same amount of unique red.

Observers were presented two rectangular patches on the CRT monitor driven by the VSG 2/5 graphical board. Both patches were spatially homogeneous in colour and surrounded by a neutral background. One patch was a reference stimulus. As reference stimuli, we used four unique hues preliminary established individually for each observer. The second patch was a variable stimulus adjusted by observers so that it looked as having the same amount of the unique hue as the reference stimulus. In a single trial an observer could change only the excitation purity of the variable stimulus, its dominant wavelength being fixed. Throughout the experiment the dominant wavelength varied at 10–15 levels (depending on the unique hue).

The isochromes were found to be essentially nonlinear. If one interprets isochromes as the equal-response curve of the channel producing the unique hues, then it follows that these channels are non-linear. Moreover, we found symmetry between neither R- and G-isochromes, nor Y- and B-isochromes. It follows that, contrary to the general view, the channel producing the unique hues are unipolar, i.e., non-opponent.